Cancerous cells contain numerous mutations that can result in recognition of the cells by a host's immune system. Appreciation of this phenomenon has prompted much research into potential immunotherapies to harness the host's immune system for attacking cancer cells. Eliminating these cells or reducing them to a level that is not life-threatening has been a major goal, as reviewed in Maraveyas, A. & Dalgleish, A. G. 1997 Active immunotherapy for solid tumours in vaccine design in The Role of Cytokine Networks, Ed. Gregoriadis et al., Plenum Press, New York, pages 129-145; Morton, D. L. and Ravindranath, M. H. 1996 Current concepts concerning melanoma vaccines in Tumor Immunology—Immunotherapy and Cancer Vaccines, ed. Dalgleish, A. G. and Browning, M., Cambridge University Press, pages 241-268.
Such work in the cancer immunotherapy field can be classified into five categories, non-specific immunotherapy, antibodies and monoclonal antibodies, subunit vaccines, gene therapy, and cell-based vaccines.
Non-Specific Immunotherapy
Efforts to stimulate the immune system non-specifically date back over a century to the pioneering work of William Coley (Coley, W. B., 1894 Treatment of inoperable malignant tumours with toxins of erysipelas and the Bacillus prodigosus. Trans. Am. Surg. Assoc. 12: 183). Although successful in a limited number of cases (e.g. BCG (i.e. bacille Calmette-Guérin) for the treatment of urinary bladder cancer, IL-2 for the treatment of melanoma and renal cancer) it is widely acknowledged that non-specific immunomodulation is unlikely to prove sufficient to treat the majority of cancers. While non-specific immune-stimulants may lead to a general enhanced state of immune responsiveness, they lack the targeting capability and also subtlety to deal with tumour lesions which have many mechanisms and plasticity to evade, resist and subvert immune-surveillance.
Antibodies and Monoclonal Antibodies
Passive immunotherapy in the form of antibodies, and particularly monoclonal antibodies, has been the subject of considerable research and development as anti-cancer agents. Originally hailed as the magic bullet because of their exquisite specificity, monoclonal antibodies have failed to live up to their expectation in the field of cancer immunotherapy for a number of reasons, including immune responses to the antibodies themselves and inability of the antibody to access the lesion through the blood vessels (thereby abrogating their activity). To date, few products have been registered as pharmaceuticals for human use, notably Rituxan (IDEC/Genentech/Hoffman la Roche) and Herceptin (Genentech/Hoffman la Roche) with over 50 other projects in the research and development pipeline. Antibodies also may be employed in active immunotherapy utilizing anti-idiotype antibodies which appear to mimic (in an immunological sense) cancer antigens. Although elegant in concept, the utility of antibody-based approaches may ultimately prove limited by the phenomenon of ‘immunological escape,’ where a subset of cancer cells in a mammalian or human subject mutates and loses the antigen recognized by the particular antibody and thereby can lead to the outgrowth of a population of cancer cells that are no longer treatable with that antibody.
Subunit Vaccines
Drawing on the experience in vaccines for infectious diseases and other fields, many researchers have sought to identify antigens that are exclusively or preferentially associated with cancer cells, namely tumour specific antigens (TSA) or tumour associated antigens (TAA), and to use such antigens or fractions thereof as the basis for specific active immunotherapy.
There are numerous ways to identify proteins or peptides derived therefrom which fall into the category of TAA or TSA. For example, it is possible to utilize differential display techniques whereby RNA expression is compared between tumour tissue and adjacent normal tissue to identify RNAs which are exclusively or preferentially expressed in the lesion. Sequencing of the RNA has identified several TAA and TSA which are expressed in that specific tissue at that specific time, but therein lies the potential deficiency of the approach in that identification of the TAA or TSA represents only a “snapshot” of the lesion at any given time which may not provide an adequate reflection of the antigenic profile in the lesion over time. Similarly a combination of cytotoxic T lymphocyte (CTL) cloning and expression-cloning of cDNA from tumour tissue has lead to identification of many TAA and TSA, particularly in melanoma. The approach suffers from the same inherent weakness as differential display techniques in that identification of only one TAA or TSA may not provide an appropriate representation of a clinically relevant antigenic profile.
Over fifty subunit vaccine approaches are in development for treating a wide range of cancers, although none has yet received marketing authorization for use as a human pharmaceutical product. In a similar manner to that described for antibody-based approaches above, subunit vaccines also may be limited by the phenomenon of immunological escape.
Gene Therapy
Most gene therapy trials in humans concern cancer treatment. A substantial proportion of these trial have purported to trigger and/or amplify patients' immune responses. Of particular note in are Allovectin-7 and Leuvectin, developed by Vical Inc for a range of human tumours, and StressGen Inc.'s stress protein gene therapy for melanoma and lung cancer. It is too early to judge whether these and the other ‘immuno-gene therapies’ in development by commercial and academic bodies ultimately will prove successful. However the commercial utility of these approaches are expected to be more than a decade away.
Cell-Based Vaccines
Tumours have the remarkable ability to counteract the immune system in a variety of ways. These include, downregulating the expression of potential target proteins; mutation of potential target proteins; downregulating surface expression of receptors and other proteins; downregulating MHC class I and II expression thereby hindering direct presentation of TAA or TSA peptides; downregulating co-stimulatory molecules leading to incomplete stimulation of T-cells and thus to anergy; shedding of selective, non representative membrane portions that act as decoys to the immune system; shedding of selective membrane portions that anergise the immune system; secreting inhibitory molecules; inducting T-cell death; and other ways. Because of this wide diversity of escape mechanisms, their immunological heterogeneity and plasticity, tumours growth has to be matched with suitable immunotherapeutic strategies that can account for such heterogeneity. The potential advantages are:                (a) whole cells contain a broad range of antigens, providing an antigenic profile of sufficient heterogeneity to match that of the lesions as described above;        (b) being multivalent (i.e. containing multiple antigens), the risk of immunological escape is reduced (the probability of cancer cells ‘losing’ all of these antigens is remote); and        (c) cell-based vaccines include TSAs and TAAs that have yet to be identified as such; it is possible if not likely that currently unidentified antigens may be clinically more relevant than the relatively small number of TSAs/TAAs that are known.        
Cell-based vaccines fall into two categories. The first category uses autologous cells. Typically a procedure within this category begins with taking a biopsy from a patient, cultivating tumour cells from the biopsy in vitro, modifying the cultivated cells through transfection and/or other means, irradiating the modified cells to render them replication-incompetent, and then injecting the replication-incompetent cells back into the same patient as a vaccine. Although this approach enjoyed considerable attention over the past decade, it has been increasingly apparent that this individually-tailored therapy is inherently impractical for several reasons. The procedure is time consuming as the lead time for producing clinical doses of vaccine often may exceed the patients' life expectancy. The procedure may be expensive and, as a ‘bespoke’ product, it is not possible to specify a standardised product (only the procedure, not the product, can be standardised and hence optimised and quality controlled). Still further, the tumour biopsy used to prepare the autologous vaccine generally will have unique growth characteristics, interactions and communications with surrounding tissue. The characteristics of the initial cell sample, which reflect a particular environment at a single time point from a tumour may severely limit the use of autologous cells for immunotherapy, wherein a vaccine desirably may be administered over the entire presentation time of a disease.
The second category of cell-based vaccines utilize allogeneic cells. These vaccines comprise cells that that genetically (and hence immunologically) are mismatched to patients. Allogeneic cell procedures benefit from the same advantages of multivalency as autologous cells. In addition, allogeneic cell vaccines can utilize immortalized cell lines, which can be cultivated indefinitely in vitro. Thus, this approach overcomes the lead-time and cost disadvantages of autologous methodologies.
Numerous publications extol the utility of cell-based cancer vaccines. See, for example, Dranoff, G. et al. WO 93/06867; Gansbacher, P. WO 94/18995; Jaffee, E. M. et al. WO 97/24132; Mitchell, M. S. WO 90/03183; and Morton, D. M. et al. WO 91/06866. These studies report procedural variations that range from a basic technique of using cancer cells as an immunotherapy antigen, to transfecting the cells to produce GM-CSF, IL-2, interferons or other immunologically-active molecules to the use of ‘suicide’ genes. Various research groups have reported the use of allogeneic cell lines for use against melanoma, that are HLA-matched or partially-matched to a patients' haplotype and allogeneic cell lines that are mismatched to the patients' haplotype. Also described are mismatched allogeneic prostate cell lines transfected with GM-CSF.
Despite this intensive work in a crucial field of medical science, successful and reproducible eradication or inhibition of cancer growth remains elusive. Any new material or procedure that can address and at least partially overcome the limitations inherent in the use of cell based vaccines would provide very important benefits for treatment of this disease. These needs are satisfied by the present invention.